1. Field of the Invention
The invention relates to a telecommunication network comprising a main station coupled to a plurality of sub-stations over a channel, each sub-station comprising an auxiliary modulator for generating an auxiliary signal by modulation of an input signal on a subcarrier having a frequency that is different for each sub-station, and a modulator for deriving a modulated transmit signal from the auxiliary signal.
The invention likewise relates to a sub-station to be used in a telecommunication network defined above.
2. Description of the Related Art
A telecommunication network as defined in the opening paragraph is known from the journal article "Optical Interference in Subcarrier Multiplexed Systems with Multiple Optical Carriers" by C. Desem in IEEE Journal on Selected Areas in Communication, Vol. 8, No. 7, September 1990, pp. 1290-1295.
Such transmission systems are used for communication between a main station and a plurality of sub-stations over a channel used in common by the substations. This channel may be formed, for example, by a glass fibre, a coaxial cable or a radio link. Applications of such transmission systems are, for example, passive optical networks, local area networks, satellite communication systems and mobile telephone systems.
In telecommunication networks utilizing a common channel for all the sub-stations it is to be ensured that no or only little mutual interference is caused by sub-stations simultaneously transmitting information to the main station.
Generally, this may be achieved by giving the signals to be transmitted by the sub-stations their own frequencies, so that at the main station, for example, by means of frequency selective filters, the signals coming from different sub-stations may be separated. Another option is making a different time slot in a frame available to each sub-station, so that only a single sub-station at a time transmits a signal to the main station.
A simple alternative to said methods is the telecommunication network known from said journal article, in which sub-stations are used which generate a transmit signal with a carrier frequency that may be about the same for the various sub-station This carrier frequency is modulated, for example, in amplitude (or intensity), frequency or phase by an auxiliary signal which itself is obtained by modulation of an input signal on a subcartier which is of different frequencies for the various sub-stations.
The receiver in the main station comprises a demodulator followed by separating means for separating the signals coming from the different sub-stations. These separating means may comprise, for example, band filters or synchronous detectors followed by low-pass filters.
Said journal article describes that interference may be expected from other sub-stations on the signal coming from a specific sub-station. This interference is caused by interference of different carriers having a substantially equal frequency in the receiver of the main station. The intensity of this interference depends, among other things, on the bandwidth of the auxiliary signal, the spectral bandwidth of the carrier, on differences of the carrier frequencies of different sub-stations and on the number of sub-stations. The interference increases with the number of sub-stations and decreases as the frequency difference between different carriers and the spectral bandwidth of the carrier increase.
In practice the maximum data transfer rate of such prior-art communication network is restricted by this mutual interference caused by signals coming from different sub-stations.